The present invention relates to a reflecting/refractive optical system. More particularly, the present invention relates to a long-focal length, super-large aperture ratio lens system suitable for use either in an astrocamera or as a supe-telephoto lens system.
Besides having a long focus, optical systems suitable for use in astrocameras are required to meet the following conditions:
(1) a super-large aperture ratio of F.sub.NO 1:2.8 - 4 that enables a fast photographic speed;
(2) a satisfactorily small chromatic aberration; and
(3) spherical abberation, coma and astigmatism that are compensated to the marginal area.
Conventional optics that satisfy these requirements include:
(A) refractive optics that employ super-low dispersion glass;
(B) reflecting optics that employ parabolic reflecting surfaces; and
(C) reflecting/refractive optics in which reflecting surfaces are combined with refractive lens systems.
Optics of type (A) are described in Japanese Unexamined Published Patent Application Nos. 220711/1984 and 14613/1987. However, long-focus and large-aperture ratio optics of this type have the following problems:
(i) the optical material used, especially super-low dispersion glass, is very expensive;
(ii) very heavy weight;
(iii) chromatic aberration cannot be satisfactorily compensated if the aperture ratio is on the order of F.sub.NO 1:2.8.
An example of the optica of type (B) is a "Newtonian" reflecting system. However, Newtonian reflecting optics having aperture ratios in the range of F.sub.NO 1:2.8 - 4 have the following problems:
(i) Coma at off-axis points is too large to ensure the necessary image circle; and
(ii) Difficulties involved in producing a parabolic surface results in a low production rate.
An example of the optics of type (C) is a "Schmidt" system, which, however, has the following problems:
(i) It is difficult to work a corrector plate having a biguadratic aspherical surface; and
(ii) Because of a curved film surface (imaging surface), the imaging apparatus is inconvenient to operate.
To solve the aforementioned problems, a variety of optical systems have been proposed, but they still have the problem of production problem chiefly because of the use of aspherical surfaces.
Reflecting/refractive optics are also available for use as photographic lens systems, but they are composed of too many elements and their aperture ratio, which is on the order of F.sub.NO 1:5.6-11, is insufficient to be used for astrocameras.